Treatment of mammalian cells with the endoplasmic reticulum-proliferator compactin strongly induces recombinant and endogenous xenobiotic metabolizing enzymes and 3-hydroxy-3-methylglutaryl-CoA reductase in vitro.

Some xenobiotics induce membrane-bound drug metabolizing enzymes (Xme) and a profound proliferation of the endoplasmic reticulum (ER) in vivo. However these effects are much weaker in vitro, possibly due to absence of certain transcription factors. We tested the possibility that ER proliferation can affect the level of ER-resident enzymes even in the absence of transcriptional activation. For this purpose we analysed the effects of compactin, which has been shown to induce ER proliferation in vitro, on recombinant Xme, which were expressed from a constitutive viral promoter. High levels of recombinant UDP-glucuronosyltransferase UGT1A6 were achieved by amplification of the UGT1A6 cDNA using the dihydrofolate reductase cDNA as selectable marker in DHFR- CHO cells. Treatment of the resulting cell lines with lipoprotein-deficient serum in the absence and presence of compactin for 5 days resulted in a 1.3- and 2.3-fold, respectively, increase of the UGT enzyme activity towards 4-methylumbelliferone, paralleled by an induction of immunoreactive UGT1A6 protein. Similarly, treatment with this 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor increased the endogenous P450 reductase activity 2.6-fold, concomitant with an increase of immunodetectable protein. As expected compactin induced the level of 3-hydroxy-3-methylglutaryl-CoA reductase. Increased levels of this protein have been associated with a proliferation of the ER. Compactin treatment of a separate cell line that expressed recombinant human P450 reductase increased this enzyme activity fivefold. Pulse-chase experiments revealed that the induction of the recombinant Xme by compactin was most likely due to decreased protein degradation. Our results show that enzyme systems unrelated to those involved in cholesterol biosynthesis are affected by compounds known to affect membrane biogenesis. Since this effect extends to heterologously expressed enzymes, it also provides an efficient means by which to increase the levels of recombinant ER proteins.